This invention relates generally to the handling of ladders, and especially to aid devices facilitating the transport of heavy ladders and the raising thereof.
A ladder, which is a structure for climbing up and down, consists of two sidepieces or rails joined at intervals by crosspieces or rungs on which one may step. In order to reach heights in excess of fifteen feet, use is often made of multi-section, extendable ladders.
Extension ladders are relatively heavy and cumbersome and they are difficult to handle even in the collapsed state in which the sections overlie each other. The difficulties experienced in carrying and raising extension ladders is severely compounded in windy, inclement weather. Attempts have been made to reduce these difficulties by fabricating them from lightweight materals, such as aluminum. There are, however, many instances where the use of electrically-conductive, light-weight materials can pose severe safety hazards, as for example in the proximity of overhead electrical feeder cables.
On the other hand, electrically non-conductive, high structural strength ladders, such as those constructed from plastic rerinforced fiber glass, have a weight comparable to wood. While such ladders are electrically safe and obviate problems of structural degradation due to weather conditions, they fail because of their weight and size to solve the handling problem. If, therefore, one needs to bring a multi-section ladder to a work site, whether the ladder is of light-weight metal, wood or fiber glass, it can take two men to perform this task and then to lift the ladder from the ground to a properly raised position against a wall.
Because of the high cost of professional labor and household assistance, there is currently a strong do-it-yourself trend in the United States. Thus many jobs such as house painting and roof repair, which call for ladders, are performed by the homeowner and in some instances by women. The manipulation of ladders by do-it-yourselfers represents a serious problem and may be hazardous, particularly if only a single person is available to carry out this task, as is generally the case.
While, in the past, women had not been expected to make use of ladders, the movement toward equality in all fields has reached a point where women are now employed in manual jobs heretofore reserved to men and requiring the use of ladders. This is particularly true in the public utilities fields of telephone and electricity supply where there is presently heavy emphasis by the federal government on the equal employment of women in outside crafts jobs, such as linesmen.
Thus, should an individual of modest strength find it necessary to transport a multi-section ladder, say from a garage at the rear of a house to a work site at the front of the house, the individual may strain himself in carrying out this task. And even should the individual succeed in getting the ladder to the site, he is then faced with the problem of lifting the ladder from a horizontal to a vertical position, and then to an inclined position against a wall.
If we assume a collapsed ladder composed of two over-lapped fourteen foot sections, and a worker who is say five feet, seven inches tall, it will be appreciated that the raising of this ladder from a horizontal position may be quite troublesome, for at some stage the worker will find himself below the inclined ladder holding onto a rung less than midway from the ground end of the ladder. The worker must now from this position of poor mechanical advantage, try to elevate the ladder to a vertical position and he may find that he lacks the strength to do so.
The physical act of exerting force upon the ladder is not a simple one in which the worker merely pushes upwards. Once the ladder is above his head, the worker is required to exert some component of muscular effort in a forward direction as he walks forward while continuing to raise the ladder. Under certain circumstances, depending upon the physical stature of the worker, a situation may arise in which the total, resultant force the worker is called upon to exert actually exceeds the weight of the ladder by an appreciable amount.
It is also to be noted that it is necessary for a worker of modest stature to rest the lower end of the ladder against some fixed abutment. By exerting the horizontal component of raising force against this abutment the worker avoids acting as a fulcrum himself. Then, as he proceeds to walk towards the lower end of the ladder, he passes below the center of gravity of the ladder. Thus at one point in the course of the raising operation, the ladder will tend to turn about the point of application of the raising force. Should the lower end of the ladder slip at this critical moment, bodily injury may occur.
In order to more fully appreciate this problem, one must take into account elementary principles of mechanics and in particular the factors of mechanical advantage and work. In a lever, mechanical advantage defines the ability of any available force to overcome a resisting force and depends on the point relative to the fulcrum at which the available force is applied.
Considering now a ladder lying on the ground which is to be raised to a vertical position, the fulcrum is the lower or ground end of the ladder, the available force is the strength of the individual seeking to raise the ladder, while the resisting force is the ladder load. At the outset, when the individual grasps the high end of the ladder to lift it from the ground, the mechanical advantage has its greatest value, for the point at which the available force is now applied is at the maximum distance from the fulcrum.
But since the worker is not nearly tall enough to fully raise the ladder, he must, as the high end of the ladder goes through an upward arc, move under the ladder in the direction of the low end of the fulcrum. The closer he gets to the low end, the poorer becomes the mechanical advantage and the greater the strength then necessary to effect a lifting action. And, as previously stated, should he be of such a stature that he proceeds below the center of gravity while raising the ladder, then, unless the lower end of the ladder is fixed to the ground in some manner, then the ladder will rotate about the point of application of the lifting force.
To express the problem in terms of work, which in physics equals the applied force times the distance, the force is, of course, dependent on the strength of the individual handling the ladder and is therefore limited, but the distance depends on the point at which the force is applied. Hence the distance traversed by the point at which the force is applied is greatest at the high end of the ladder and is reduced as one shifts this point toward the fulcrum or low end. It therefore requires much more physical strength to produce the work necessary to raise the ladder when the force is applied at a point near the fulcrum than when it is applied near the high end of the ladder.
Similarly, to carry a heavy ladder in conventional fashion, the individual must lift the ladder from the ground and he therefore must have sufficient strength therefor. Moreover, the point at which he holds the ladder represents a fulcrum and unless the ladder's weight on either side of the fulcrum is balanced, the ladder will tip. As a practical matter, the size and weight of most sectional ladders are such as to make it hard for a man of modest strength and stamina to carry it, to say nothing of a woman's ability to do so, for it is a well-accepted principle of the sciences of biomechanics and human engineering that a woman possesses about 55 percent of a man's strength in the arm and shoulder muscles, and about 80 percent of a man's strength in the muscles in the hip and lower leg.